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Carbon Arc Cutting (With Diagram) | Metallurgy

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Metal cutting may be done by oxy-acetylene flame—this is called Flame-cutting or Oxy-Gas cutting. An arc cutting is essentially a method of applying the heat energy of the electric arc to melt away the metal along a desired line of cut.

Because of its high temperature and low cost of heat production, the arc is especially suitable for cutting of those metals which cannot be readily oxidised—cast-iron, manganese steel, and non-ferrous metals.

To cut metals with carbon electrodes and with coated metallic electrodes use different processes. In arc cutting, there is no action tending to force the molten metal out of the cut itself. Some provision must be made to permit the molten metal to flow out of the cut. The cut should generally be started at same point from which the molten metal may readily flow by the force of gravity.

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Arc cutting is inferior to flame cutting—both in quality and rate of cutting. Application for arc cutting include production cutting of cast iron, alloy steels, and non-ferrous metals and scrap cutting. Carbon, graphite or metallic electrodes may be used.

Carbon electrodes are mainly used for all arc cutting. Graphite electrodes permit higher current concentration, remain sharp for a long time, and produce a neater kerf (notch or groove) than carbon electrodes. Cutting is carried on with direct current, with the electrode on the negative side of the circuit.

In the carbon tire cutting of plates up to 12 mm in thickness, the arc is simply advanced along the section to be cut. The cutting may be done in any position, but best results are obtained when the ‘work’ is in a horizontal plane or in a vertical position in an upward direction to permit the liquid metal to flow readily out of the cut.

The angularity of the electrode to the work depends upon the position of the ‘work’. For vertical cutting, the electrode should be pointed downward. For flat cutting, the electrode may be held either vertical, or slightly forward (Fig. 15.7).

Metal arc cutting is superior to carbon arc cutting because the cut is narrower and a smoother surface of the kerf is obtained; and alternating current may be used. In metal arc cutting, preference should be given to coated electrodes.

The core material may be any low-carbon steel which is not suitable for welding. With coated electrodes, a short arc should be made, and the electrode should be kept pushing closely up in the slot. The upward motion should be very fast, and the downward motion should supply a pushing effect and thus help to force out the molten metal and slag.

The coating slows down the melting of the electrode, stabilizes the arc and isolates the core from the work as the electrode is being fed into the cut. Arc cutting may be performed with metal electrodes and with sources of current supply used for arc welding (Fig. 15.8).

Coated electrodes are often used to pierce holes in plates. After the arc struck at the point where the hole is desired, the electrode should be moved in a circular motion equal to the diameter of the proposed hole until the metal is melted. The electrode is then pushed down against the molten metal, while the circular motion is continued. As the operation continues the electrode melts a hole through the plate (Fig. 15.9).